GM:
As B Company formed up inside the earthen fort, like most every unit in the Army of the Potomac, they were arranged by height with the tallest at the front of the column. Randolph, Harris and Donald were all men of towering height, each over 6 feet tall and always in the front rank with Randolph the tallest by an inch in the center, flanked by his brother and Donald. About the only man taller in the entire regiment was the Colonel himself, at almost six and half feet...
Dobs and the Bartons are all in the 6'1" - 6'2" range. The average American soldier in 1865 was 5'8", so Dobs and the Bartons are tall, but not exceptionally so. (The average American male is 5'9" today, and the Barton brothers' heights are consistent with RL.)
But "six and a half feet' would be 6'6" (i.e., much taller), so I assume, based on the full context, the colonel is actually 6 feet and one half inch? Just making sure we're all on the same page...
FWIW, the US President during the Civil War, Abraham Lincoln, was a little over 6'3", but he probably looked 6'11" with his top hat. :)